/// \file mingw.shared_mutex.h
/// \brief Standard-compliant shared_mutex for MinGW
///
/// (c) 2017 by Nathaniel J. McClatchey, Athens OH, United States
/// \author Nathaniel J. McClatchey
///
/// \copyright Simplified (2-clause) BSD License.
///
/// \note This file may become part of the mingw-w64 runtime package. If/when
/// this happens, the appropriate license will be added, i.e. this code will
/// become dual-licensed, and the current BSD 2-clause license will stay.
/// \note Target Windows version is determined by WINVER, which is determined in
/// <windows.h> from _WIN32_WINNT, which can itself be set by the user.

//  Notes on the namespaces:
//  - The implementation can be accessed directly in the namespace
//    mingw_stdthread.
//  - Objects will be brought into namespace std by a using directive. This
//    will cause objects declared in std (such as MinGW's implementation) to
//    hide this implementation's definitions.
//  - To avoid poluting the namespace with implementation details, all objects
//    to be pushed into std will be placed in mingw_stdthread::visible.
//  The end result is that if MinGW supplies an object, it is automatically
//  used. If MinGW does not supply an object, this implementation's version will
//  instead be used.

#ifndef MINGW_SHARED_MUTEX_H_
#define MINGW_SHARED_MUTEX_H_

#if !defined(__cplusplus) || (__cplusplus < 201103L)
#error A C++11 compiler is required!
#endif

#include <cassert>

//    Use MinGW's shared_lock class template, if it's available. Requires C++14.
//  If unavailable (eg. because this library is being used in C++11), then an
//  implementation of shared_lock is provided by this header.
#if(__cplusplus >= 201402L)
#include <shared_mutex>
#endif
//  For defer_lock_t, adopt_lock_t, and try_to_lock_t
#include "mingw.mutex.h"

//  For descriptive errors.
#include <system_error>
//    Implementing a shared_mutex without OS support will require atomic read-
//  modify-write capacity.
#include <atomic>
//  For timing in shared_lock and shared_timed_mutex.
#include <chrono>

//  For this_thread::yield.
#include "mingw.thread.h"

//  Might be able to use native Slim Reader-Writer (SRW) locks.
#ifdef _WIN32
#include <windows.h>
#endif

namespace mingw_stdthread
{
  //  Define a portable atomics-based shared_mutex
  namespace portable
  {
    class shared_mutex
    {
      typedef uint_fast16_t counter_type;
      std::atomic< counter_type > mCounter;
      static constexpr counter_type kWriteBit = 1
          << (sizeof(counter_type) * CHAR_BIT - 1);

#if STDMUTEX_RECURSION_CHECKS
      //  Runtime checker for verifying owner threads. Note: Exclusive mode
      //  only.
      _OwnerThread mOwnerThread;
#endif
     public:
      typedef shared_mutex* native_handle_type;

      shared_mutex()
          : mCounter(0)
#if STDMUTEX_RECURSION_CHECKS
          , mOwnerThread()
#endif
      {
      }

      //  No form of copying or moving should be allowed.
      shared_mutex(const shared_mutex&) = delete;
      shared_mutex&
      operator=(const shared_mutex&) = delete;

      ~shared_mutex()
      {
        //  Terminate if someone tries to destroy an owned mutex.
        assert(mCounter.load(std::memory_order_relaxed) == 0);
      }

      void
      lock_shared(void)
      {
        counter_type expected = mCounter.load(std::memory_order_relaxed);
        do
        {
          //  Delay if writing or if too many readers are attempting to read.
          if(expected >= kWriteBit - 1)
          {
            using namespace std;
            using namespace this_thread;
            yield();
            expected = mCounter.load(std::memory_order_relaxed);
            continue;
          }
          if(mCounter.compare_exchange_weak(expected, expected + 1,
                                            std::memory_order_acquire,
                                            std::memory_order_relaxed))
            break;
        } while(true);
      }

      bool
      try_lock_shared(void)
      {
        counter_type expected =
            mCounter.load(std::memory_order_relaxed) & (~kWriteBit);
        if(expected + 1 == kWriteBit)
          return false;
        else
          return mCounter.compare_exchange_strong(expected, expected + 1,
                                                  std::memory_order_acquire,
                                                  std::memory_order_relaxed);
      }

      void
      unlock_shared(void)
      {
        using namespace std;
#ifndef NDEBUG
        if(!(mCounter.fetch_sub(1, memory_order_release) & (~kWriteBit)))
          throw system_error(make_error_code(errc::operation_not_permitted));
#else
        mCounter.fetch_sub(1, memory_order_release);
#endif
      }

      //  Behavior is undefined if a lock was previously acquired.
      void
      lock(void)
      {
#if STDMUTEX_RECURSION_CHECKS
        DWORD self = mOwnerThread.checkOwnerBeforeLock();
#endif
        using namespace std;
        //  Might be able to use relaxed memory order...
        //  Wait for the write-lock to be unlocked, then claim the write slot.
        counter_type current;
        while(
            (current = mCounter.fetch_or(kWriteBit, std::memory_order_acquire))
            & kWriteBit)
          this_thread::yield();
        //  Wait for readers to finish up.
        while(current != kWriteBit)
        {
          this_thread::yield();
          current = mCounter.load(std::memory_order_acquire);
        }
#if STDMUTEX_RECURSION_CHECKS
        mOwnerThread.setOwnerAfterLock(self);
#endif
      }

      bool
      try_lock(void)
      {
#if STDMUTEX_RECURSION_CHECKS
        DWORD self = mOwnerThread.checkOwnerBeforeLock();
#endif
        counter_type expected = 0;
        bool ret = mCounter.compare_exchange_strong(expected, kWriteBit,
                                                    std::memory_order_acquire,
                                                    std::memory_order_relaxed);
#if STDMUTEX_RECURSION_CHECKS
        if(ret)
          mOwnerThread.setOwnerAfterLock(self);
#endif
        return ret;
      }

      void
      unlock(void)
      {
#if STDMUTEX_RECURSION_CHECKS
        mOwnerThread.checkSetOwnerBeforeUnlock();
#endif
        using namespace std;
#ifndef NDEBUG
        if(mCounter.load(memory_order_relaxed) != kWriteBit)
          throw system_error(make_error_code(errc::operation_not_permitted));
#endif
        mCounter.store(0, memory_order_release);
      }

      native_handle_type
      native_handle(void)
      {
        return this;
      }
    };

  }  //  Namespace portable

//    The native shared_mutex implementation primarily uses features of Windows
//  Vista, but the features used for try_lock and try_lock_shared were not
//  introduced until Windows 7. To allow limited use while compiling for Vista,
//  I define the class without try_* functions in that case.
//    Only fully-featured implementations will be placed into namespace std.
#if defined(_WIN32) && (WINVER >= _WIN32_WINNT_VISTA)
  namespace vista
  {
    class condition_variable_any;
  }

  namespace windows7
  {
    //  We already #include "mingw.mutex.h". May as well reduce redundancy.
    class shared_mutex : windows7::mutex
    {
      //    Allow condition_variable_any (and only condition_variable_any) to
      //    treat a
      //  shared_mutex as its base class.
      friend class vista::condition_variable_any;

     public:
      using windows7::mutex::lock;
      using windows7::mutex::native_handle;
      using windows7::mutex::native_handle_type;
      using windows7::mutex::unlock;

      void
      lock_shared(void)
      {
        AcquireSRWLockShared(native_handle());
      }

      void
      unlock_shared(void)
      {
        ReleaseSRWLockShared(native_handle());
      }

//  TryAcquireSRW functions are a Windows 7 feature.
#if(WINVER >= _WIN32_WINNT_WIN7)
      bool
      try_lock_shared(void)
      {
        return TryAcquireSRWLockShared(native_handle()) != 0;
      }

      using windows7::mutex::try_lock;
#endif
    };

  }     //  Namespace windows7
#endif  //  Compiling for Vista
#if(defined(_WIN32) && (WINVER >= _WIN32_WINNT_WIN7))
  using windows7::shared_mutex;
#else
  using portable::shared_mutex;
#endif

  class shared_timed_mutex : shared_mutex
  {
    typedef shared_mutex Base;

   public:
    using Base::lock;
    using Base::lock_shared;
    using Base::try_lock;
    using Base::try_lock_shared;
    using Base::unlock;
    using Base::unlock_shared;

    template < class Clock, class Duration >
    bool
    try_lock_until(const std::chrono::time_point< Clock, Duration >& cutoff)
    {
      do
      {
        if(try_lock())
          return true;
      } while(std::chrono::steady_clock::now() < cutoff);
      return false;
    }

    template < class Rep, class Period >
    bool
    try_lock_for(const std::chrono::duration< Rep, Period >& rel_time)
    {
      return try_lock_until(std::chrono::steady_clock::now() + rel_time);
    }

    template < class Clock, class Duration >
    bool
    try_lock_shared_until(
        const std::chrono::time_point< Clock, Duration >& cutoff)
    {
      do
      {
        if(try_lock_shared())
          return true;
      } while(std::chrono::steady_clock::now() < cutoff);
      return false;
    }

    template < class Rep, class Period >
    bool
    try_lock_shared_for(const std::chrono::duration< Rep, Period >& rel_time)
    {
      return try_lock_shared_until(std::chrono::steady_clock::now() + rel_time);
    }
  };

#if __cplusplus >= 201402L
  using std::shared_lock;
#else
  //    If not supplied by shared_mutex (eg. because C++14 is not supported), I
  //  supply the various helper classes that the header should have defined.
  template < class Mutex >
  class shared_lock
  {
    Mutex* mMutex;
    bool mOwns;
    //  Reduce code redundancy
    void
    verify_lockable(void)
    {
      using namespace std;
      if(mMutex == nullptr)
        throw system_error(make_error_code(errc::operation_not_permitted));
      if(mOwns)
        throw system_error(
            make_error_code(errc::resource_deadlock_would_occur));
    }

   public:
    typedef Mutex mutex_type;

    shared_lock(void) noexcept : mMutex(nullptr), mOwns(false)
    {
    }

    shared_lock(shared_lock< Mutex >&& other) noexcept
        : mMutex(other.mutex_), mOwns(other.owns_)
    {
      other.mMutex = nullptr;
      other.mOwns = false;
    }

    explicit shared_lock(mutex_type& m) : mMutex(&m), mOwns(true)
    {
      mMutex->lock_shared();
    }

    shared_lock(mutex_type& m, defer_lock_t) noexcept : mMutex(&m), mOwns(false)
    {
    }

    shared_lock(mutex_type& m, adopt_lock_t) : mMutex(&m), mOwns(true)
    {
    }

    shared_lock(mutex_type& m, try_to_lock_t)
        : mMutex(&m), mOwns(m.try_lock_shared())
    {
    }

    template < class Rep, class Period >
    shared_lock(mutex_type& m,
                const std::chrono::duration< Rep, Period >& timeout_duration)
        : mMutex(&m), mOwns(m.try_lock_shared_for(timeout_duration))
    {
    }

    template < class Clock, class Duration >
    shared_lock(mutex_type& m,
                const std::chrono::time_point< Clock, Duration >& timeout_time)
        : mMutex(&m), mOwns(m.try_lock_shared_until(timeout_time))
    {
    }

    shared_lock&
    operator=(shared_lock< Mutex >&& other) noexcept
    {
      if(&other != this)
      {
        if(mOwns)
          mMutex->unlock_shared();
        mMutex = other.mMutex;
        mOwns = other.mOwns;
        other.mMutex = nullptr;
        other.mOwns = false;
      }
      return *this;
    }

    ~shared_lock(void)
    {
      if(mOwns)
        mMutex->unlock_shared();
    }

    shared_lock(const shared_lock< Mutex >&) = delete;
    shared_lock&
    operator=(const shared_lock< Mutex >&) = delete;

    //  Shared locking
    void
    lock(void)
    {
      verify_lockable();
      mMutex->lock_shared();
      mOwns = true;
    }

    bool
    try_lock(void)
    {
      verify_lockable();
      mOwns = mMutex->try_lock_shared();
      return mOwns;
    }

    template < class Clock, class Duration >
    bool
    try_lock_until(const std::chrono::time_point< Clock, Duration >& cutoff)
    {
      verify_lockable();
      do
      {
        mOwns = mMutex->try_lock_shared();
        if(mOwns)
          return mOwns;
      } while(std::chrono::steady_clock::now() < cutoff);
      return false;
    }

    template < class Rep, class Period >
    bool
    try_lock_for(const std::chrono::duration< Rep, Period >& rel_time)
    {
      return try_lock_until(std::chrono::steady_clock::now() + rel_time);
    }

    void
    unlock(void)
    {
      using namespace std;
      if(!mOwns)
        throw system_error(make_error_code(errc::operation_not_permitted));
      mMutex->unlock_shared();
      mOwns = false;
    }

    //  Modifiers
    void
    swap(shared_lock< Mutex >& other) noexcept
    {
      using namespace std;
      swap(mMutex, other.mMutex);
      swap(mOwns, other.mOwns);
    }

    mutex_type*
    release(void) noexcept
    {
      mutex_type* ptr = mMutex;
      mMutex = nullptr;
      mOwns = false;
      return ptr;
    }
    //  Observers
    mutex_type*
    mutex(void) const noexcept
    {
      return mMutex;
    }

    bool
    owns_lock(void) const noexcept
    {
      return mOwns;
    }

    explicit operator bool() const noexcept
    {
      return owns_lock();
    }
  };

  template < class Mutex >
  void
  swap(shared_lock< Mutex >& lhs, shared_lock< Mutex >& rhs) noexcept
  {
    lhs.swap(rhs);
  }
#endif  //  C++11
}  //  Namespace mingw_stdthread

namespace std
{
//    Because of quirks of the compiler, the common "using namespace std;"
//  directive would flatten the namespaces and introduce ambiguity where there
//  was none. Direct specification (std::), however, would be unaffected.
//    Take the safe option, and include only in the presence of MinGW's win32
//  implementation.
#if(__cplusplus < 201703L) \
    || (defined(__MINGW32__) && !defined(_GLIBCXX_HAS_GTHREADS))
  using mingw_stdthread::shared_mutex;
#endif
#if(__cplusplus < 201402L) \
    || (defined(__MINGW32__) && !defined(_GLIBCXX_HAS_GTHREADS))
  using mingw_stdthread::shared_lock;
  using mingw_stdthread::shared_timed_mutex;
#elif !defined(MINGW_STDTHREAD_REDUNDANCY_WARNING)  //  Skip repetition
#define MINGW_STDTHREAD_REDUNDANCY_WARNING
#pragma message \
    "This version of MinGW seems to include a win32 port of\
 pthreads, and probably already has C++ std threading classes implemented,\
 based on pthreads. These classes, found in namespace std, are not overridden\
 by the mingw-std-thread library. If you would still like to use this\
 implementation (as it is more lightweight), use the classes provided in\
 namespace mingw_stdthread."
#endif
}  //  Namespace std
#endif  // MINGW_SHARED_MUTEX_H_